In common use are many types of miniature portable appliances, such as cassette players, compact disc players, or credit card-sized radios, which, through a helmet or a pair of headphones, allow the user to listen to favorite music or programs anywhere and at anytime. Such appliances have enjoyed great success for several years but they have the major disadvantage of acoustically isolating the user from the surrounding environment, particularly since a large number of users prefer to listen at a very high volume. Although perfect acoustic isolation allows for listening comfort, such isolation could subject the user to a wide range of accidents or risks.
Indeed, it is observed everyday and in every environment, such as in parks, on highways, or in sports grounds, that the majority of users of portable appliances are, by virtue of their helmets or headphones, completely insensitive to calls, warning signals, and, above all, to any sudden, unexpected change in their surroundings. This drawback could have such severe consequences that, in many locales, the use of such appliances with helmets or headphones is strictly forbidden to drivers of vehicles, pedestrians, and others who are required to be alert to their surroundings.
Furthermore, it is unpleasant, when calling someone nearby who is wearing a helmet or headphones, to have to shout in order to warn the person of some danger or simply to attract the person's attention.
Consequently, the use of such appliances in public places or places which are heavily frequented is not recommended because it renders the users insensitive to calls or communications of general or safety interests. On the other hand, such appliances are frequently used in noisy environments, such as outside where cars, buses, or aircraft produce high ambient sound levels, or inside, where factory machinery, vacuum cleaners, air conditioning systems, and people talking produce high ambient sound levels.